Cephalosporin-resistance in the Bacteroides fragilis group and the effect of clavulanic acid.

Thirty-five beta-lactam-resistant isolates of the Bacteroides fragilis group were examined for the susceptibility to eight cephalosporins, alone, and in combination with a subinhibitory concentration of the beta-lactamase inhibitor, clavulanic acid. The majority of strains tested became fully susceptible to cephaloridine, cefotaxime and ceftriaxone in the presence of clavulinic acid whereas the effect of the inhibitor on the susceptibility to compounds such as cefsulodin and ceftazidime and to a lesser extent cefoperazone varied according to species. Bact. fragilis strains almost invariably showed substantial reductions in MICs whereas those of other species, especially Bact. thetaiotaomicron, were influenced to a lesser extent. Susceptibility to cefoxitin was unaffected by the inhibitor but a few strains with high level resistance to latamoxef (moxalactam) became susceptible in the presence of clavulanic acid. All strains were found to produce beta-lactamase as determined by the nitrocefin test and these were characterized by isoelectric focusing.